1. Field of the Invention
The present invention relates to devices for determining the coordinates (i.e., X and Y coordinates) of a location in a two-dimensional system such as touch sensitive screens for producing output signals related to a touched position. The present invention more particularly provides a 5-wire (or 9-wire) resistive touch sensor having improved linearity near the periphery of the touch sensitive area.
2. Description of the Prior Art
A touchscreen is a transparent input device that can sense the two-dimensional position of the touch of a finger or other electronically passive stylus. Touchscreens are placed over display devices such as cathode-ray-tube monitors and liquid crystal displays to provide inputs for restaurant order entry systems, industrial process control applications, interactive museum exhibits, public information kiosks, lap-top computers, etc.
Presently, the dominant touch technologies are 4-wire resistive, 5-wire resistive, capacitive, ultrasonic, and infrared. These are technologies that have delivered high standards of performance at cost-competitive prices. An important aspect of touchscreen performance is a close correspondence between true and measured touch positions at all locations within the touch sensitive area.
5-wire resistive touchscreens, e.g. the AccuTouch(trademark) product line of Elo TouchSystems, Inc. of Fremont, Calif., have been widely accepted for many touchscreen applications. In these touchscreens, mechanical pressure from a finger or stylus causes a plastic membrane coversheet to flex and make physical contact with an underlying glass substrate. The glass substrate is coated with a resistive layer upon which voltage gradients are excited. Via electrical connections to the four corners of the coated glass substrate, associated electronics can sequentially excite gradients in both the X and Y directions, as described in U.S. Pat. No. 3,591,718. The underside of the coversheet has a conductive coating that provides an electrical connection between the touch location and voltage sensing electronics. Since both X and Y voltage gradients are generated on the substrate""s resistive coating, the coversheet coating need only provide electrical continuity. Further details regarding 5-wire resistive touchscreens are found U.S. Pat. No. 4,220,815 to Gibson; U.S. Pat. Nos. 4,661,655 and 4,731,508 to Gibson et al.; U.S. Pat. No. 4,822,957 to Talmadge et al.; U.S. Pat. No. 5,045,644 to Dunthorn; and U.S. Pat. No. 5,220,136 to Kent.
Electronics can obtain touch information from a 5-wire resistive touchscreen via current injection as well as the voltage excitation described above. For current injection read out, a current source injects current though the coversheet and the currents arriving at each of the four corner connection points are then measured during a touch. From the sums and ratios of these corner currents, touch positions are reconstructed. The choice between current injection and voltage excitation is an electronics design choice and is largely independent of touchscreen design. Peripheral electrode pattern designs for touch systems with voltage-excitation electronics are equally applicable to touch systems with current-injection electronics.
Capacitive touchscreens often require peripheral electrode patterns that serve the same basic function as in 5-wire resistive touchscreens. MicroTouch Systems, Inc. offers both capacitive touchscreens (ClearTek(trademark)) and 5-wire resistive touchscreens (TouchTek(trademark)) with peripheral electrode patterns similar to FIG. 1b of U.S. Pat. No. 4,371,746 of Pepper. In a capacitive touchscreen, the coversheet is replaced by a thin transparent dielectric coating that then forms the exterior surface of the ITO or ATO coated substrate. An oscillating voltage is applied to the four corner connection points. A finger touch provides an AC shunt to ground and hence serves as an AC current source (sink) at the location of the touch. The division of this AC current between the four corner connection points is measured and used to determine touch coordinates. An AC variant of current-injection electronics is used.
It is sometimes advantageous to have both a drive and a sense line connection between the electronics and each of the four corner connection points. With appropriate feedback loops in the electronics, the combination of drive and sense lines gives the electronics better control over the excitation voltages applied to the corner connection points. This leads to a variant of xe2x80x9c5-wirexe2x80x9d touchscreens with 9 wire connections between the electronics and the touchscreen. The design of the peripheral electrode pattern is largely unaffected by the choice between 5-wire and 9-wire connection schemes.
A 5-wire resistive touchscreen typically includes a glass substrate 1 to 3 mm thick on which a transparent resistive coating has been applied. A peripheral electrode pattern is formed on the substrate as a geometric pattern of printed conductive ink on the resistive coating, and insulating regions formed in the resistive coating. ITO (indium tin oxide) and ATO (antimony tin oxide) are examples of degenerate semi-conductors which have the important property of being both conductive and transparent, and may serve as resistive coatings. The regions between the conductive electrodes form resistors, with the insulating regions defining conductive paths therebetween. Details of the electrode pattern and its manufacture are found in U.S. application Ser. No. 08/989,928 (allowed), which is incorporated herein by reference. This information is also found in published PCT application No. WO99/30272.
U.S. Pat. Nos. 3,591,718 (Asane and Baxter), 4,198,539 (Pepper), and 4,797,514 (Talmadge and Gibson) disclose peripheral electrode patterns that include a continuous resistive electrode between corner connection points. The performance of such electrode patterns is sensitive to the stability and uniformity of the resistivity of the electrodes. Non-uniformity of the electrical properties of a continuous electrode along its length will distort the linearity of the sensor. Humidity and temperature variations in the application environment may have different effects on the electrical properties of continuous resistive electrodes, e.g. fabricated from a printed composite polymer ink, and the resistive coating in the touch area, e.g. ITO. If so, the linearity of the touchscreen may be compromised.
This sensitivity is in contrast to peripheral electrode patterns using discrete overlap resistors such as those found in Elo TouchSystems"" AccuTouch(trademark) products and disclosed in U.S. Pat. No. 5,045,644 to Dunthorn, which is incorporated herein by reference. Provided that good electrical contact is made between the electrode and the resistive coating, the resistance of discrete overlap resistors is dominated by the resistance of the resistive coating in the gap between the electrodes. The resistance of the electrodes printed with conductive inks is small in comparison, and hence there is little effect if the electrical properties of the conductive inks vary. Furthermore, as the gap resistance is formed of the same coating as used in the touch area, it will track variations of active region resistivity as a function of temperature and humidity. This provides the touchscreen with stable linearity even if the ohms/square of the resistive coating changes with environmental conditions.
Use of discrete overlap resistors leads to a discrete set of parallel connections between the peripheral electrode pattern and the touch sensitive region. This produces a ripple non-linearity in the touch area near the peripheral electrode pattern. An example of ripple can be seen in FIG. 4B of U.S. Pat. No. 5,045,644. Equipotential lines for both X and Y excitations are shown. Consider the set of equipotential lines used for X coordinate measurements. In the lower left corner, towards the center of the touch area and far from the peripheral electrode pattern, the X equipotential lines appear as uniformly spaced vertical lines. However, on the right side of this figure, distortion of the X equipotential lines is quite apparent. A finger moving down a vertical straight line (with a constant true X coordinate) in this region will experience variations in the X excitation voltage and hence variations in the measured X coordinate (unless otherwise corrected for). The cursor movement will be subject to perpendicular ripple distortion. There is a need to reduce this ripple while preserving the benefits of peripheral electrode designs that use overlap resistors.
Referring again to the right side of FIG. 4B of U.S. Pat. No. 5,045,644, note that the ripple tends to be approximately periodic with one repetition per overlap resistor electrode. Let S be a representative spacing of the overlap resistors and their electrodes along the series resistor chain. Then as a function of the vertical coordinate y, the potential in the ripple region for the X excitation will vary roughly sinusoidally with a wavelength of S, i.e., have approximate functional form cos(2xcfx80*y/S). As is well known in electrostatics, the voltage excitation pattern in the touch area obeys the mathematics of Laplace""s equation. To satisfy Laplace""s equation, the X excitation potential within the ripple region at the right and left sides of the touch area will have approximately the following form.
A* exp(xc2x12xcfx80*x/S)* cos(2xcfx80*y/S)
The plus or minus in the exponent is minus if the X axis is directed away from the peripheral electrode border causing the ripple. Here A is a normalization constant. A similar expression applies to the Y excitation potential for the ripple near the top and bottom regions of the touch area.
Note that in the X direction, the above expression exponentially decays with a decay length of S/2xcfx80. Here we refer to the distance at which the ripple amplitude decays by a factor of e (2.71828 . . . ) as the xe2x80x9cripple decay lengthxe2x80x9d. A short ripple decay length is desirable to maximize the region of the touch area with little or no ripple. This in turn suggests reducing S by increasing the number of overlap resistors in the series resistor chains. As discussed below, however, the number of overlap resistors cannot be arbitrarily increased.
In the peripheral electrode patterns such as those disclosed in U.S. Pat. No. 5,045,644 and U.S. Ser. No. 08/989,928, one may attempt to reduce ripple by increasing the number of overlap resistors and hence increase the density of parallel connections to the touch area. However, if the number of overlap resistors in a series resistor chain is doubled, there is approximately half the perimeter length available with which to construct each overlap resistor. For a given gap distance, this roughly doubles the resistance of each overlap resistor. Combined with a doubling of the total number of overlap resistors, the total series resistance of the chain is quadrupled. As this total resistance increases, it rapidly becomes more difficult for the peripheral electrode pattern to maintain linear equipotential lines in the touch area. There remains a need to further reduce ripple, even after the number of overlap resistors in the resistor chain has been maximized.
Historically, most touch applications involve user activation of software programmed touch buttons or touch zones. A moderate amount of ripple non-linearity about the perimeter of the touch region typically had no noticeable effect. However touchscreens are now used in a variety of applications which make use of graphical user interfaces (GUI) that place small area buttons, scroll bars, and pull-down menus near the perimeter of the displayed image. If a finger is dragged in a vertical straight line along a narrow scroll bar, it is important that the reconstructed touch coordinates also follow a straight line. Perpendicular ripple distortion may cause problems for the operation of scroll bars and pull down menus. Parallel ripple distortion is less important here as it will not cause the cursor to leave the scroll bar or pull-down menu. Note that GUI interfaces will often place a narrow scroll bar close to perimeter of the touch zone.
U.S. Pat. No. 4,371,746 (Pepper) illustrates another approach to minimizing ripple non-linearity in peripheral electrode patterns incorporating discrete overlap resistors. FIG. 1b of U.S. Pat. No. 4,371,746 shows an electrode pattern with four successive rows of electrodes of increasing granularity. The granularity of the electrodes becomes finer closer to the touch area. The outer course grained electrodes provide the desired value of resistance between adjacent corner connection points while a succession of inner electrode rows of finer granularity can be continued until a desired reduction of ripple is achieved in the touch area. However, compared to patterns using a single series resistor chain between adjacent corner electrodes, a relatively wide peripheral electrode pattern is needed. Due to the need for narrow borders in touchscreen designs, there remains a need to reduce ripple in designs using a single series resistor chain between adjacent corner electrodes.
To achieve linear touchscreen system performance, it is not a requirement that the touchscreen itself be linear. Non-linear distortions can be corrected in either the electronics or by software in the host computer. For example, see U.S. Pat. No. 5,940,065 (Babb and Wilson). Smoothly varying deviations from linearity in the touchscreen are relatively easy to correct, while rapidly varying non-linearities such are ripple are much more problematic. Hence, even in touch systems with a non-linear touchscreen, there is a need to reduce ripple, thus leaving non-linearities that can more easily be corrected by downstream signal processing.
It is a principal object of this invention to reduce ripple non-linearity in a resistive or capacitive touchscreen having a substrate with four corner connection points. It is a related object to reduce the ripple non-linearity while maintaining a border electrode pattern which is relatively narrow so that the touch area is maximized.
These objects are achieved by providing a single series resistor chain of overlap resistor electrodes between each pair of corners, and by providing multiple parallel connections to each overlap resistor electrode as a means to reduce ripple non-linearity. In a preferred embodiment, this is accomplished by providing an insulating pattern proximate and interior to the resistor chain. The insulating pattern comprises a plurality of adjacent insulating regions in a linear array. The insulating regions are created by a lack of the resistive layer. Adjacent insulating regions are separated by gaps. The insulating pattern is positioned between the electrodes and the touch area. Such gaps may be referred to herein as deletion gaps. These gaps provide the current paths for the parallel connections between the touch area and the electrodes. Note that deletion gaps per se are known from U.S. Ser. No. 08/989,928, but that application only discloses a single deletion gap per overlap resistor electrode, resulting in a sinusoidal ripple potential based on an electrode spacing S as discussed above.
In the case of two parallel deletion gap connections per overlap resistor electrode, the ripple repeats with a distance Sxe2x80x2 equal to half the spacing of the overlap resistor electrodes. Hence the exponential decay length of the ripple is Sxe2x80x2/(2xcfx80)=S/(4xcfx80), thus greatly reducing the size of the ripple region. In addition to this effect, numerical simulations demonstrate an equally dramatic reduction in the amplitude Axe2x80x2 in the expression Axe2x80x2* exp(xc2x12xcfx80*x/Sxe2x80x2)* cos(2xcfx80*y/Sxe2x80x2). The net effect is to significantly reduce ripple distortion. Three or more parallel connections can further reduce ripple distortion.
The scope of this invention includes not only deletion gaps placed along the electrodes, but also deletion gaps placed between the junction between overlap resistor electrodes and the touch area. Simulations show that this design is equally effective in suppressing perpendicular ripple distortion. However, simulations indicate that designs with such junction deletion gaps tend to perform slightly less well in suppressing parallel ripple distortion.
In many touchscreen applications, there is limited space available for a touchscreen border region outside the touch area. This invention minimizes this border region in two ways.
First, by using only a single series resistor chain, the width of the electrode region can be kept small. For example, using xe2x80x9cZxe2x80x9d shaped electrodes as in FIG. 8 of WO99/30272 provides a narrow series resistor chain. The parallel connections provided by gaps in a fine broken deletion line add little to the width of the peripheral electrode pattern. Second, the penetration of unacceptably large ripple non-linearity into the touch area is greatly reduced. The net effect is a greatly reduced border width with no added manufacturing cost.
As with the present AccuTouch(trademark) product from Elo, the resistances of the parallel connections can be varied to provide a linear touchscreen. Shrinking the deletion gap increases the resistance while increasing the deletion gap dimension decreases the resistance of the parallel connection. Linear sensor designs can be iteratively obtained by building and testing designs, or by development of formulas of effective electrode resistance based on simulation of the component electrodes.
The invention provides improved geometry of conductive, resistive, and insulating regions of the peripheral electrode pattern. This improved geometry is not limited to any particular choice of materials or electronic read out methods.